I think GK heads are basically as described by 006-- very solid and great bang for the buck. I actually recomended a 400rb at work for a budget backline rig. What I don't like is that many of their heads are built for weird bi- or tri-amp configurations that no one really wants. For example the 1001RB has 700watts low and 50 watts high. What that means in practice is that 99% of people are wasting the 50 watt channel. This is totally fine-- just a little silly. Still, I don't think you can beat the GK heads for value.
Honestly I'm not sure I've even seen one of the cabs in use before. I've definitely never played one. I know that GK has an 8x10 here for around $600 but I'm honestly skeptical you could get 8 good speakers for that amount of $-- let alone a whole cab. I don't know UK prices but for $1k in the US the ampeg fridge is hard to argue with. It pretty much defined the rock sound (and look) so if you can swing it I think you should.
All of the other major players -- SWR, Eden, and Aguilar run around $1500 stateside for an 8x10. The aggie is honestly the best cab I've ever played, but it is expensive and even heavier than the ampeg. It is also true that the aggie, swr, and eden all have more full range and less mid focused sounds than the ampeg, which I prefer but which is certainly different.
I could go on forever but hopefully this helps some. If you can find and play a GK cab and it sounds good then obviously you should ignore my speculation.
Good bang for your buck bass amps IMO. 
